I was having serious problems with emscripten and fullscreen mode - in particular, clicking on the standard 'go fullscreen' button in the standard html page wasn't resizing the canvas properly, or notifying my app that window had been resized.

I eventually got it working by tweaking emscriptenevents.c like this...

EM_BOOL
Emscripten_HandleFullscreenChange(int eventType, const EmscriptenFullscreenChangeEvent *fullscreenChangeEvent, void *userData)
{
/*make sure this is actually our element going fullscreen*/
/* removed by mark!
if(SDL_strcmp(fullscreenChangeEvent->id, "SDLFullscreenElement") != 0)
return 0;
*/
SDL_WindowData *window_data = userData;
if(fullscreenChangeEvent->isFullscreen)
{
//added by mark!
window_data->windowed_width=window_data->window->w;
window_data->windowed_height=window_data->window->h;

SDL_bool is_desktop_fullscreen;
window_data->window->flags |= window_data->requested_fullscreen_mode;

if(!window_data->requested_fullscreen_mode)
window_data->window->flags |= SDL_WINDOW_FULLSCREEN_DESKTOP; /*we didn't reqest fullscreen*/

window_data->requested_fullscreen_mode = 0;

is_desktop_fullscreen = (window_data->window->flags & SDL_WINDOW_FULLSCREEN_DESKTOP) == SDL_WINDOW_FULLSCREEN_DESKTOP;

/*update size*/
if(window_data->window->flags & SDL_WINDOW_RESIZABLE || is_desktop_fullscreen)
{
emscripten_set_canvas_size(fullscreenChangeEvent->screenWidth, fullscreenChangeEvent->screenHeight);
SDL_SendWindowEvent(window_data->window, SDL_WINDOWEVENT_RESIZED, fullscreenChangeEvent->screenWidth, fullscreenChangeEvent->screenHeight);
}
else
{
/*preserve ratio*/
double w = window_data->window->w;
double h = window_data->window->h;
double factor = SDL_min(fullscreenChangeEvent->screenWidth / w, fullscreenChangeEvent->screenHeight / h);
emscripten_set_element_css_size(NULL, w * factor, h * factor);
}
}
else
{
/* removed by mark!
EM_ASM({
//un-reparent canvas (similar to Module.requestFullscreen)
var canvas = Module['canvas'];
if(canvas.parentNode.id == "SDLFullscreenElement") {
var canvasContainer = canvas.parentNode;
canvasContainer.parentNode.insertBefore(canvas, canvasContainer);
canvasContainer.parentNode.removeChild(canvasContainer);
}
});
*/
double unscaled_w = window_data->windowed_width / window_data->pixel_ratio;
double unscaled_h = window_data->windowed_height / window_data->pixel_ratio;
emscripten_set_canvas_size(window_data->windowed_width, window_data->windowed_height);

if (!window_data->external_size && window_data->pixel_ratio != 1.0f) {
emscripten_set_element_css_size(NULL, unscaled_w, unscaled_h);
}

SDL_SendWindowEvent(window_data->window, SDL_WINDOWEVENT_RESIZED, unscaled_w, unscaled_h);
window_data->window->flags &= ~FULLSCREEN_MASK;
}

return 0;
}

Note that I had to comment out code that referenced 'SDLFullscreenElement', and added some code to 'save' the window size before going fullscreen. The latter I think is due to the fact I use SDL_SetWindowSize, and that doesn't seem to update windowed_width/windowed_height anywhere, so when I returned from fullscreen window size was wrong.

This is probably not the right way to fix this stuff, but it should be enough for someone who knows what they're doing to fix it properly.

I *think* I'm using the latest version of everything - em++ -v says '1.35.0' and the version of SDL I'm using is from a github mirror of hg.libsdl.org/SDL.

And sorry about code formatting, this is the first time I've bugzilla'd.

Bye!
Mark